1. Field of the Invention
This invention concerns devices used to deflect an optical beam through an angle using an acoustic beam. It also concerns spectrum analyzers in which this type of device analyzes the electrical signal spectrum used to obtain ultrasounds which interact with the light beam to modify its frequency.
2. Discussion of the Related Art
FIG. 1 shows a known type of such a device, normally called a Bragg cell, which includes a crystal with anisotropic elasto-optical properties cut out in the shape of a rectangular parallelepiped 101.
Based on this crystal, an electro-acoustic transducer 102 is excited by an electrical signal at frequency F.sub.O. The ultrasonic signal thus generated forms a beam which propagates in the crystal to form a stationary waves system 103, schematically shown on the figure by a series of lines. The optical properties of the crystal are modified at the peaks and nodes of these stationary waves by the elasto-optical properties of the material used.
An incident light beam I with optical aperture W designed to interact with acoustic waves, arrives on one of the large faces of part 101 at an angle .THETA..sub.i. Under the refraction effect at its entry into block 101, it intercepts the acoustic beam 103 at angle .THETA..sub.B, called the Bragg's angle. The incident beam divides into two parts under the effect of the interaction between the light waves and the ultrasonic waves, one part I.sub.0 which is not affected by this interaction and which finally crosses the cell as it would cross a slide with parallel faces, and the other part I.sub.1 which is affected by the interaction and which leaves the ultrasonic beam at an angle .epsilon. very slightly inclined relative to the normal to this beam and therefore, in this previously known technique, relative to the cell entry and exit faces. This beam I.sub.1 leaves the cell at an angle Hd itself low with respect to the normal to these faces of the cell, simply by the application of the laws of refraction.
The light wave of beam I.sub.1 affected by the acousto-optical infraction has a frequency different from the frequency of the incident beam I, whereas the beam I.sub.0 has the same frequency as the incident beam I. This beam I.sub.0 is therefore useless and corresponds to a parasite fraction which should be minimized as much as possible, although can never be completely eliminated.
To prevent the beam I.sub.0 from reflecting diffracting on the lower side of the cell as it leaves the interaction area, thus forming parasite light which could disturb the measurements made on the beam I.sub.1, arrangements are made so that the lower limit of the incident beam I strikes the cell at a distance d from its lower face such that the lower part of the beam I.sub.0 does not strike this lower face and that the entire beam I.sub.0 leaves through the cell exit face (through which I.sub.1 normally leaves).
The orientation of the crystal from which block 101 is carved, and the angle of incidence .THETA..sub.i, are selected, by a known method, so that this cubicle works in a mode called "Bragg tangential", for which there are two frequencies f.sub.0, one low and one high, for which the tolerance on the device phase match is maximum. This makes it possible to use the device on a relatively wide band around these frequencies f.sub.0. In known practice, this corresponds to an anisotropic type acousto-optical interaction taking advantage of the optical diffraction fringes of some crystals such as Li Nb O.sub.3, with so-called tangential "Bragg" type phase match, allowing two possible frequency choices for the same acoustic direction.
The set of parameters corresponding to these operating conditions is such that the output angle .epsilon. (and therefore .THETA..sub.d) is low, as described above.
The band width thus available on the frequency f.sub.0 makes it possible to use this type of device to analyze the spectrum of an electrical signal with a relatively wide bandwidth around f.sub.0. A diagram of this type of analyzer is shown in FIG. 2.
A coherent and parallel light emitter 204, for example a laser diode, emits the beam I with optical width W which is applied to cell 101. The electrical signal to be analyzed, with central frequency f.sub.0 is applied to the transducer 102 for this cell and, as described above, an output beam I.sub.1 results, affected by the acousto-optical interaction.
The frequency of this beam I.sub.1 depends on the frequency f.sub.0, but in fact this is not the phenomenon that is used here. In fact, as described above, the output angle .THETA..sub.d of beam I.sub.1 is low, but it also depends on the frequency f.sub.0 ; this is the effect that is used as described below.
This beam I.sub.1 is applied to a convergent lens 205 which focuses on a multiple detector 206 formed from a network of elementary photoelectric cells which have distinct outputs.
Under these conditions, for each distinct frequency included in the complex signal applied to transducer 102, a distinct luminous spot is obtained on one of the cells in the network 206. This luminous spot also has an intensity approximately proportional to the intensity of the spectral beam corresponding to the distinct frequency. We thus obtain an electrical signal on each of the electrical outputs of analyzer 206, the amplitude of which corresponds to the corresponding spectral line contained in the signal to be analyzed.
In order to eliminate the optical signal I.sub.0 not affected by the interaction, an absorbent screen 207 is used which intercepts the signal so that there is no risk of it reflecting arbitrarily and disturbing the measurements.
However, the optical width W of the incident beam I, used as a basis to attempt to eliminate reflections of beam I.sub.0 on the bottom of the cell, is only a theoretical value which in no way corresponds to a sudden beam cutoff. In fact, it is impossible to obtain this type of clean cut, and a gradual attenuation of the light intensity is observed beyond the limits fixed by the aperture W. This is known under the name of Gaussian drag.
For various reasons related to size and ultrasound absorption, it is impossible to apply the beam I sufficiently high on the cell 101 entry face for this Gaussian drag to be reduced sufficiently to prevent its reflection on the lower face of the cell from disturbing the measurements. Under these conditions, there is actually a parasite reflection, or at least a diffusion, on this lower face. This generates a parasite beam I.sub.R which is not well delimited, and which passes through lens 205 to distribute a diffused signal on detector 206. This diffused signal obviously disturbs the measurements, which has the same effect as increasing the overall background noise.
An optic-acoustic modulator using the same interaction effect between the light wave and the ultrasonic wave has been described elsewhere in French patent 2 635 879 registered on Aug. 25, 1989 in the name of the Litton Systems company. However, the various parameters and particularly the interaction cell entry and exit angles, are selected such that the exit beam is colinear with the entry beam. In this system, obviously the frequency change of the emerging wave is used rather than its direction change which is deliberately made almost zero and invariable with the frequency. The choice of these parameters also implies that the electrical signal used to modulate the optical signal can never be wideband.